linux-kernel - Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET

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Message-ID: <c206865e-b2da-b996-3d48-2c71d7783fbc@redhat.com>
Date:   Mon, 7 Dec 2020 12:29:34 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     stsp <stsp2@...dex.ru>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6]
 KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)

On 07/12/20 12:24, stsp wrote:
> It tries to enable VME among other things.
> qemu appears to disable VME by default,
> unless you do "-cpu host". So we have a situation where
> the host (which is qemu) doesn't have VME,
> and guest (dosemu) is trying to enable it.
> Now obviously KVM_SET_CPUID doesn't check anyting
> at all and returns success. That later turns
> into an invalid guest state.
> 
> 
> Question: should KVM_SET_CPUID check for
> supported bits, end return error if not everything
> is supported?

No, it is intentional.  Most bits of CPUID are not ever checked by KVM, 
so userspace is supposed to set values that makes sense or just copy the 
value of KVM_GET_SUPPORTED_CPUID more or less blindly.

Paolo